Preheat the oven to 190C/375F/Gas 5. Line a baking tray with greaseproof paper.
Beat the butter and sugar together in a bowl until combined.
Beat in the egg and vanilla extract, a little at a time, until well combined.
Stir in the flour until the mixture comes together as a dough.
Roll the dough out on a lightly floured work surface to a thickness of 1cm/½in.
Using Halloween-style biscuit cutters, cut biscuits out of the dough and carefully place onto the baking tray.
Bake the biscuits for 8-10 minutes, or until pale golden-brown. Set aside to harden for 5 minutes, then cool on a wire rack.
For the icing, sift the icing sugar into a large mixing bowl and stir in enough water to create a smooth mixture. Stir in the food colouring.
Carefully spread the icing onto the biscuits using a knife and set aside until the icing hardens.